The present invention relates to the production of very accurately plane sheets of small uniform thickness which are in the form of a sheet or strip of great length. In the description below the term "films" will be used to describe such products.
Such films, produced particularly in glass possess numerous practical applications, among which the following are examples:
The protection of plastic materials in sheet form. Certain plastics materials are limited in their use, because their surface is sensitive to scratching and to abrasion effects, or because fatty substances adhere to them, so that they are difficult to keep clean. For this use, a glass film may be applied upon the surface of the plastics sheet. A type of glass which is resistant to abrasion and to chemical attack will preferably be used for this purpose;
Joining two sheets of plastics to form composite, stratified materials. By contrast with the preceding example, the glass film will generally not be the surface of the composite material, and glass having other mechanical properties, particularly good modulus of elasticity and tensile strength, will be chosen;
The production of automobile windscreens. When glass is used for this application, it is absolutely essential that this material shall not, if it is broken, produce fragments which are capable of causing cuts or which may enter the skin or the eyes. The use of very thin glass for the rear layer of the windscreen may assist in conferring desirable fragmentation properties upon it;
The production of panes which reflect infra-red radiations. In the panes currently produced, the reflective layer is placed outside in order to prevent heating, but this arrangement leaves the reflective layer exposed to abrasion and to the weather. The use of a film of glass enables the reflective layer to be placed upon the internal surface of the film without the risk of heating up, and with the production of an excellent protective effect;
The easy and economic production of surface forms described as "ruled" due to the great flexibility of the film-type glass which enables it to adapt, by simple bending, to these forms.
Using conventional methods for the production of plane glass films, it is possible only with extreme difficulty to obtain plane sheets of thicknesses less than 500.mu., at acceptable production yields.
Certain special processes do give a film-type glass by drawing. The uniformity, planeness and the regime of internal stresses of such glasses are however not satisfactory. These faults not only adversely effect the commercial quality of the product, but in addition they make it difficult to handle the glass and to store it either in stacks or in rolls, without damaging the film.
Processes of evaporation under vacuum are also known. These enable a film of glass to be deposited, which adheres directly to the pieces of plastics material to be protected. The results obtained by such methods are however not satisfactory for dimensions in excess of a few centimeters since, under the effect of physical or chemical action, the plastics and the glass undergo differential expansion and the result is a loss of adhesion or even rupture of the glass layer.
The production of thin films may have advantages for numerous materials other than glass, for example for metals. It is known that thin metal deposits may be produced on sheets of glass, plastics, or other metals. Moreover, very thin sheets of ductile metals of small dimensions may also be obtained by using various methods (such as rolling, hammering etc.). The production of films of large dimensions of certain metals or metallic oxides, where these films are independent of any support, can however involve very great difficulties. However, such a product may possess very great practical importance if it is desired to store it in film form, in order to apply it later to a support of a different material.